A typical photographic element contains multiple layers of light sensitive photographic silver halide emulsions with one or more of these layers being spectrally sensitized to each of blue light, green light and red light. In the conventional subtractive color system, the blue, green and red light sensitive layers typically contain yellow, magenta and cyan dye forming couplers, respectively.
To form color photographic images, the color photographic material is exposed imagewise and processed in a color developer bath containing an aromatic primary amine color developing agent. Image dyes are formed by the coupling reaction of these couplers with the oxidized product of the color developing agent.
It has been an ongoing object of photographic researchers efforts for many years to develop and combine cyan, magenta and yellow image dyes of different chemical structures in order to improve the range of colors produced and hence increase the dye color gamut.
Direct viewing color print materials such as color papers, transparent back-lighted advertising materials, motion picture print films and color reversal slide films rely on the formation of color metamers within the photographic element to reproduce the color of an image. The color image is formed by generating a combination of cyan, magenta and yellow dyes in proportion to the amounts of exposure of red green and blue light respectively onto the element with the object being for the reproduced image to duplicate as nearly as possible the stimulation of the optic sensors of the eye resulting from the original image.
Thus, any color in the original scene is reproduced as a unique combination of the cyan, magenta and yellow image dyes in the viewed print material. The absolute relationship of the original color to the reproduced color is a combination of many factors. It is, however, limited by the dye gamut achievable by the multitude of combinations of cyan, magenta and yellow dyes used to generate the final image. Dye gamut is a measure of the variety of colors capable of being reproduced by the combination of dyes used to make the image.
Dye gamut is limited by many features of an imaging system. For example, dye gamut is limited by the minimum and maximum densities achievable by the photographic element, by the color purity of the individual dyes, etc. Color purity of a dye is a function of the secondary absorption of the dye, the shape of the absorption band of the dye, and its bandwidth. In addition to the individual dye characteristics, to achieve the highest color gamut, it is necessary to have cyan, magenta and yellow image dyes which have the preferred absorption maxima relative to one another: narrow bandwidth (to increase color purity) and absorption band shapes which function together to provide a maximum dye gamut.
In the measurement of color, or colorimetry, the colorimetric term chroma (C*) is a measure of the color saturation or color purity (sometimes referred to as `brilliance`). Since C* changes as a function of its lightness (L*) it is necessary to specify L* when comparing C* measurements between different examples. In order to measure C*, it is first necessary to specify the illuminant under which the subject is to be measured or viewed. It is convenient to specify a color temperature rather than a specific light source such as daylight, tungsten or fluorescent. For daylight viewing, a color temperature of 5000.degree. K. is representative of a typical daylight illuminant.
Chroma itself does not imply a given color or dye hue, but rather is a measure of the purity of a given color. As such, a value for C* is first obtained by measuring two other colorimetric terms, a* and b*. These metrics, when specified in combination, describe the color of an object, whether it be red, green, blue, etc. The measurement of a* and b* is well documented and now represents an international standard of color measurement. (The well known CIE system of color measurement was established by the International Commission on Illumination in 1931 and was further revised in 1971. For a more complete description of color measurement refer to "Principles of Color Technology, 2nd Edition by F. Billmeyer, Jr. and M. Saltzman, published by J. Wiley and Sons, 1981.)
Simply stated, a* is a measure of how green or magenta the color is (since they are color opposites) and b* is a measure of how blue or yellow a color is. From a mathematical perspective, a* and b* are determined as follows: EQU a*=500{(X/X.sub.n).sup.1/3 -(Y/Y.sub.n).sup.1/3 } EQU b*=200{(Y/Y.sub.n).sup.1/3 -(Z/Z.sub.n).sup.1/3 }
Where X, Y and Z are the tristimulus values obtained from the combination of the visible reflectance spectrum of the object, the illuminant source (i.e. 5000.degree.K.) and the standard observer function.
Once a* and b* are obtained, the value of C* may be obtained by the following equation: EQU C*=(a*.sup.2 +b*.sup.2).sup.1/2
Thus in a photographic element, as dye formation increases as a function of increasing exposure, the density of the element increases. Since L* is a measurement of lightness or darkness it changes in concert with density. Since an L* of 100 is perfectly white, there is no color. Correspondingly, an L* of 0, is perfectly black and again, there is no color. Therefore color only exists if L* has a value greater than 0 and less than 100.
The value of L* is a function of the tristimulus value Y, thus EQU L*=116(Y/Y.sub.n).sup.1/3 -16
As exposure increases on a photographic element and dye density also increases in proportion due to color development, L* decreases. C*, however, increases with exposure to a maximum value. This maximum value is a function of many variables, but is generally bounded by the Dmin and Dmax of an element and the color purity of the dye being formed.
Magenta dyes absorb green light and typically have absorption maxima near the center of the green region, or about 550 nm. The most commonly used magenta couplers are those of the pyrazolone type. The image dyes derived from these couplers have several deficiencies, including an absorption spectra having too much unwanted absorption of blue and red light which limits the gamut of the colors obtainable using this type of coupler.
In recent years, magenta couplers have been developed based on pyrazolotriazole compounds, particularly the pyrazolo[1,5-c]triazole couplers. Compared to the pyrazolone based magenta couplers, thesa pyrazolotriazole couplers have been shown to have significantly lower unwanted absorption of blue and red light and to have a narrower dye adsorption bandwidth. These pyrazolotriazole couplers have also been shown to be excellent for light and dark image stability when compared to the pyrazolones. However the absorption curve for this type of coupler has been found to provide a less-than satisfactory gamut when used in combination with the types of yellow couplers currently in use. Further, the light fade of the magenta and yellow dyes formed are less than satisfactory.
Yellow dyes absorb blue light and typically have absorption maxima of about 450 nm. The precise location of the peak absorption depends upon several other factors including the shape of its absorption band, its bandwidth and the shapes and positions of the absorption bands of the cyan and magenta dyes with which it is associated. Couplers used to form the yellow dyes in direct viewing color print materials are usually based upon acylacetanilides and most typically, alkylacylacetanilides. Benzoylacetanilides are known to have absorption bands which absorb more green light than the alkylacetanilides and therefore are not preferred in direct viewing photographic systems.
Alkylacylacetanilide couplers in which the acetanilide ring is substituted with an alkoxy group in the ortho position of the anilide ring are known to produce yellow image dyes which have an absorption maxima at shorter wavelengths than those couplers which have a halogen (i.e. Cl) or other substituent. Shifting the absorption band to shorter wavelengths increases the color saturation and resultant color purity of the dye by reducing the unwanted absorption of green light. This is therefore a preferred embodiment. A preferred subclass of these yellow couplers is a cycloalkylacylacetanilide compound. The image dyes produced from these couplers have absorption maxima at shorter wavelengths with sharp cutting bands on their long wavelength sides also resulting in higher color purity.
Cyan dyes absorb red light and typically have an absorption maximum of about 650nm. Traditionally, the cyan dyes used in color papers have had nearly symmetrical absorption bands. Such dyes have rather large amounts of unwanted absorption in the green and blue regions of the spectrum. Much effort has gone into the design of the cyan dye forming coupler used in concert with the magenta and yellow couplers described above.
Couplers used to form cyan image dyes are generally derived from naphthols and phenols, as described, for example, in U.S. Pat. Nos. 2,367,351, 2,423,730, 2,474,293, 2,772,161, 2,772,162, 2,895,826, 2,920,961, 3,002,836, 3,466,622, 3,476,563, 3,552,962, 3,758,308, 3,779,763, 3,839,044, 3,880,661, 3,998,642, 4,333,999, 4,990,436, 4,960,685, and 5,476,757; in French patents 1,478,188 and 1,479, 043; and in British patent 2,070,000.
It is a problem to be solved to provide a combination of magenta and yellow dye forming couplers that together exhibit improved dye fade resistance and an equal or increased red color gamut.